CMP is utilized for removing part of a layer of a body, typically a semiconductor wafer, to provide the body with a generally flat surface in which the remainder of the layer occupies one or more depressions that extend along the body's surface in a desired pattern. During CMP, material of the depression-occupying layer is abraded with a mechanical apparatus starting at protruding areas while suitable chemicals are applied to the layer in order to weaken it and carry away abraded particles. CMP for silicon semiconductor devices is generally described in Li et al, “Chemical Mechanical Polishing in Silicon Processing”, Semiconductors and Semimetals (Academic Press), Vol. 63, 2000.
Erosion and overpolishing are important planarity concerns in CMP. Erosion leads to the formation of depressed regions along a surface being polished. As a result, the final polished surface is commonly not as planar as desired. Erosion generally increases with increasing depression pattern density. This arises largely because CMP processes remove material of the depression-occupying layer at a considerably faster rate than the field material situated to the sides of the depression or depressions. The depression pattern density at a portion of a surface being polished is generally the ratio of the total depressed area, if any, in the surface portion to the sum of the depressed area and the intervening non-depressed area.
Overpolishing refers to performing CMP for a somewhat longer time than necessary to remove the material of the depression-occupying layer situated over the field area to the sides of the depressed area. Overpolishing is performed in order to ensure that all the material of the depression-occupying layer situated over the field area is indeed removed. As a result, the exposed surface of the remaining depression-occupying material lies below the surface of the field area. Also, the surface of the depression-occupying material at areas of high pattern density is typically more recessed than the surface of depression-occupying material at areas of low pattern density, leading to further non-planarity.
CMP processes typically reduce height differences along the final polished surfaces. However, it is often difficult to compare CMP processes to determine which process actually provides better planarity. Different CMP processes commonly terminate at different end points, especially due to overpolishing. Consequently, absolute polishing data for one CMP process often cannot be readily compared to absolute polishing data for another CMP processes. In light of this, it is desirable to have an accurate technique for comparing CMP processes.